In public mobile radio networks, such as the NMT (Nordic Mobile Telephone) system, all calls have the same priority, i.e. none of the users have rights taking precedence over the rights of other users.
In private radio networks, some of which implement the trunked method, the situation is different. In such PMR (Private Mobile Radio) networks, it is possible to control the operation of the network in such a way that users carrying out specific tasks, e.g. emergency servicing or other urgent work, have a higher priority, i.e. their high-priority calls are the first to be switched in a heavily loaded traffic situation. It is even possible to define so high a priority that an existing call is cleared down before the new call if the new call cannot otherwise be switched. In most cases, the reason for switching being prevented is the fact that all radio channels are occupied, or the fact that the called subscriber is already engaged in a call. Other resources may also in fact be occupied and prevent switching of the call.
Where two-party calls are in progress in a mobile radio system, the system--for example its mobile exchange or a database in the mobile exchange--is aware of which radio units participate in the call. On this basis, when a high-priority call is to be established to a subscriber engaged in a call, the mobile exchange can terminate the ongoing call to disengage the subscriber to whom a new call is to be established. In a group call the situation is more difficult, as the mobile exchange in the mobile radio system is not necessarily aware of which group calls given subscribers belong to, and thus the mobile exchange is not aware of which radio channel said subscribers are listening to.
A group call is a call in which the system commands several, at least three, subscribers to the same call with a single command. The groups are formed of members of a company or other user organization, and often the groups are programmed into the mobile radio and the mobile exchange is not necessarily aware of which mobile radios belong to each group. Typically one mobile radio may belong to several groups. Part of the group numbers may even be such that they may be changed very often, if necessary every morning, and an indication of these changes is not necessarily sent to the mobile exchange. For example the MPT 1327 and MPT 1343 standards do not provide for means for communicating the group numbers from the mobile radios, i.e. subscriber stations, to the mobile exchange. This makes it especially difficult to include a subscriber who is already engaged in a previously initiated group call in a high-priority call. This situation is very common in mobile radio networks, as very often specifically those subscribers who are wanted in high-priority calls already participate in prior group calls on account of the fact that said subscribers have a vital function in their organizations.
The above problem has been resolved in prior art solutions by clearing down all group calls within the area of the mobile radio system. By clearing down the group calls, the subscriber who is desired to be included in the high-priority call can in fact be found, but at the same time the initial, i.e. first, group call is terminated and it must be re-established without the party that has been included in the high-priority call. This procedure causes considerable disadvantage to other subscribers in the mobile radio system, as these must unduly interrupt their group call. When interrupted group calls are re-established, the mobile radio system is loaded by unnecessary connection establishment procedures. Furthermore, if the mobile radio system has a very high traffic level, the initial group call group may forfeit the radio channel it used, as another group call or another call may seize the channel while the first group call is interrupted and one of its subscribers is being searched for a high-priority individual call or group call.
A further drawback of the prior art solution is that if it is not desired to clear down the existing group call, the subscriber wanted for the high-priority call cannot be included in the new call to be established at all. Thus the subscriber has poor availability.